Interpretive Summary: Major crop production areas have switched from growing solely conventional crops to as much as 95% transgenic crops that protect against feeding by pest insects. However, the relationships are unclear between ecological functions of insect-feeding organisms, such as bats, and insect pest management within the transgenic crop environment. An agricultural food web model was developed and evaluated to forecast regional ecological functions of Brazilian free-tailed bats which feed on bollworms that infest cotton in the Winter Garden crop production area of Texas. The extremely large population of bats in central Texas was predicted to benefit local cotton farmers by reducing the need for 1 or 2 insecticide applications and saving $19/acre or $35/acre on conventional and transgenic cotton fields, respectively. Further, regional reduction of pest insects by bats will benefit crop protection on a transcontinental scale by reducing the number of moths that migrate to infest distant farms.

Technical Abstract:
During the past 12,000 years, agricultural systems have transitioned from natural areas to conventional agricultural regions, and recently, to genetically engineered crops. The most rapid transitional link has been from non-transgenic to transgenic crops; in a span of slightly more than a decade, major agricultural growing areas have switched from growing solely conventional crops up to 95% transgenic plantings. The relationships between ecological functions and agronomics of avian or mammalian insectivores in the transgenic environment are generally unclear although the importance of pest management service provided by insectivorous species such as the Brazilian free-tailed bat, Tadarida brasiliensis, has been acknowledged.
We developed, tested and applied a dynamic stochastic difference equation model of agricultural food webs composed of insectivores, insects, and plants to investigate insectivory effects and project ecological impacts of this rapid nonlinear change of crop cultivars. We employed the theory to forecast regional ecological functions of food webs as quantified by insect emigration from the cotton fields in the Winter Garden region of Texas; to indicate insect herbivory responses with measures of cotton boll damage; and to examine agronomics of insectivory. We corroborated the theoretical model developed for the south-central Texas region by employing independent results of field studies in North Carolina.
We demonstrate that food web ecology in the Winter Garden Texas region, especially the process of insectivory by bats, has a considerable impact on both the ecology and valuation of harvest in Bt and non-transgenic cotton crops. Harvesting of agricultural pests by insectivores may enhance the economic value of agricultural systems by reducing the frequency of required spraying and delaying the ultimate need for new pesticides. Specifically, in the Winter Garden region, the presence of an extremely large number of insectivorous bats yields a regional summer dispersion of pest insects from Bt crops that is considerably diminished from dispersion in non-transgenic crops when bats are absent. This regional reduction of pests should have an impact on agricultural herbivory on the south-to-north transcontinental scale. With a few exceptions, the agronomics of both Bt and conventional cotton production is more profitable when insectivorous bats are present.